Generally, automotive vehicles use a transmission controlling output of a power generating means so that rotational speed of the power generating means can be controlled to be appropriate for the load applied to the driving means and operating state thereof.
Transmissions used for this purpose can be classified into two types. One is a manual transmission which is manually shifted to each speed changing stage, and the other is an automatic transmission which can be automatically shifted to each speed changing stage. In recent years, the automatic transmission has been popular because of the convenience in its manipulation.
Furthermore, in the automatic transmission, various speed ratios are predetermined and the automatic shift is accomplished while various operating elements are selectively operated in accordance with the vehicle's driving conditions.
Accordingly, the conventional automatic transmission generally comprises a plurality of planetary gear units, a plurality of operating elements for controlling each elements of the planetary gear unit, and a complex control system for controlling the operating elements. This results in complicating and enlarging the structure thereof Therefore, it is impossible to commonly apply the automatic transmission specified as an fixed model to various driving means. In addition, when applying the automatic transmission to the driving means such as automotive vehicles, the big size of the automatic transmission requires a big mounting space, resulting in making it difficult to mount the specified automatic transmission into the various type of vehicle.
Furthermore, in the conventional automatic transmissions, the speed changing operation is not accomplished linearly. That is, a speed change from one stage to another stage is not continuously realized such that the engine which is the power generating device is overloaded, thereby deteriorating ride comfort by shift shock.
To solve the above described problems of the conventional automatic transmission, there has been developed infinitely variable speed transmission which can continuously accomplish the speed changing operation in accordance with the vehicle's driving condition, thereby preventing the speed changing shock to improve ride comfort as well as preventing the engine from being overloaded.
An example of this kind of transmission is disclosed in U.S. Pat. No. 4,641,548. The size of the this transmission described in the U.S. patent, however, becomes larger since it comprises a plurality of operating elements such as disks, pistons and the like. Furthermore, since the transmission is designed such that a plurality of shift mechanisms are arranged on a common axis in a series, the length of the transmission becomes longer. This makes it difficult to apply the transmission to automotive vehicles which have limited space for the mounting of parts. In addition, there is another problem in that it is not economical to apply the transmission to a practical vehicle side since the transmission is costly.